Hard disk drives are data storage devices for storing digital information or data, which can be retrieved at a later time. A hard disk drive is a type of non-volatile memory that retains stored data irrespective of whether the drive has power. A hard disk drive comprises platters or disks coated with a magnetic material for storing charges. Data is stored by placing a charge on the magnetic material. The hard disk drive also comprises one or more read and write heads. The heads are configured to store charges to, and read charges from, the magnetic material of the platter. The heads are arranged on a moving actuator arm which positions the heads over the platters at the correct location to write data to, or read data from, the platters as the platters rotate at high speeds.
The use of platters and heads permits data to be read from or written to a hard disk drive in a random-access manner. Randomly accessing data refers to reading or writing a first set of data at a first location on the platter and then a second set of data at second location on the platter without having to read or write data through all intervening locations between the first and second locations.
A disk drive typically stores data within sectors of the disk. A sector is a segment of a track, and a track is a circle of recorded data on a single platter. The sector unit is usually the smallest size of data to be stored in a hard disk drive and a data file usually consists of several sector units. A digital disk drive is a block storage device, where a disk is divided into logical blocks, which are a collection of sectors. Blocks are addressed using their logical block addresses (LBA). Reading from or writing to a digital disk drive is typically performed at the granularity of blocks.
Platters are divided into multiple co-centric circular tracks, the circular tracks radiating from the center to the edge of the platter. The tracks have a width. The tracks help identify or reference the physical location where data may be, or is, stored on the platter. The width of the tracks may correspond to the width of the read or write header. Adjacent tracks may be grouped into bands or “zones” in the case of SMR disks, with a zone consisting of many sectors.
Conventional hard disk drives store data by writing magnetic tracks parallel to one another (perpendicular recording) in a non-overlapping fashion. SMR disk drives take advantage of the fact that a read head of a disk drive is typically narrower than a write head of the drive. SMR disk drives write new tracks that partially overlap a previously written track, which leaves the previous track narrower than its original width. The disk drive can still read data from the previous track in its narrow form. The overlapping of tracks in an SMR disk allows for higher track density compared to conventional non-overlapping disks.
FIG. 1A shows a 90 degree portion of an example platter 2 of an SMR disk drive, as known in the art. The platter comprises a plurality of tracks 4, which are organized into zones 6. Adjacent zones 6 are separated by a buffer or guard region 8. An area of the platter 2, such as zone 10, may be used for disk management or other purposes. One or more read and write heads of the disk are not shown.
FIG. 1B shows a top view of a portion of a platter 100 of an SMR disk, as known in the art, showing portions of two adjacent zones, zone A 104 and zone B 110. Zone A 104 comprises tracks 102a-c. Tracks 102a-c overlap with each other in a “shingled” fashion. Tracks of a zone A 104 do not overlap, however, with tracks of any adjacent zone (here zone B 110) as zones are separated with a buffer or guard region 112. The writing of tracks 102a-c in an overlapping manner permits more tracks to be stored on a platter of a given size, which thus increases the data density of the platter and the hard disk drive.
When data is to be rewritten to the disk, a problem arises because the write head 108 of the disk is wider than the tracks. For example, in FIG. 1B, write head 108 is wider than track 102a. As a result, if the disk rewrites one or more blocks in track 102a, an adjacent part of track 102b will be overwritten as well, which destroys the data stored in the overwritten part of track 102b. Accordingly, data cannot be written to SMR disks in the same random access manner as in conventional hard disk drives.
SMR disks therefore typically impose strict rules including that writes must proceed sequentially through a zone on the disk with each sector being written exactly once. Within each zone, writes must start at the zeroth sector of the zone and proceed in order to the end of the zone. Write access to any previously written sector must be preceded by a reset of the write pointer of the zone, effectively erasing all content from the zone.
Many file systems require the ability to write to a disk drive in a random access manner, including updates to previously written data, which are performed as write-in-place updates. Accordingly, the sequential write requirements of SMR disks make them inherently incompatible with file systems that support random write access.